This invention relates generally to sheet-metal fittings of the type used in ventilation air distribution systems, and in particular to sheet metal elbows that connect round branch conduits to rectangular register boxes.
Conventional ventilation air distribution systems employ a number of rectangular ducts that may run forty feet or more from a plenum box that is coupled to an updraft furnace and blower fan assembly. The cross-sectional area of the main distribution duct is reduced by appropriate duct transition fittings, typically rectangular-to-rectangular and square-to-round to maintain constant air pressure and air flow conditions along the main distribution line.
Various take-off fittings including side coupling boots and wye and tee connectors allow branch conduits to be routed laterally away from and below the main distribution duct to reach remotely located ceiling register boxes. The branch conduits are typically made of galvanized sheet-metal, but sometimes are made of a flexible, wire-reinforced plastic sheet material. Sheet-metal elbows having 45.degree. and 90.degree. bends are available for connecting the branch conduits to the air inlet panel of a ceiling register box.
Conventional elbows have a crimped neck portion that fits within a female coupling collar attached to the flat air inlet panel of the register box. The female coupling collar is attached to the inlet panel by insertion of the circular inlet panel portion that borders the air inlet opening into a radial groove formed in the coupling collar. The cross-sectional air flow area of the elbow is matched to the size of the branch conduit to avoid an excessive pressure drop.
The elbow should be initially adjustable through 360.degree. with respect to the register box to accommodate various branch conduit take-off run angles. The rotatable seal should have negligible air leakage at the interface union. Moreover, the inside flow surface through the elbow and across the elbow/register box union should be smooth and continuous, providing a substantially constant flow area cross-section and minimum flow resistance through the union. The union should be structurally stable for supporting the hanging load of the branch conduit, and also so that the elbow will not push into the register box or separate from the register box.
According to conventional construction practice, the main distribution ducts are suspended by straps from roof joists and the branch conduits are run over to pre-cut ceiling openings in which air distribution register boxes are to be mounted. Snap-on rails are usually provided for attaching the register boxes onto ceiling joists, with the register boxes being suspended from the joists and fitting into the pre-cut holes. The lower edge of the register box is inset slightly from the ceiling dry board panel for engaging an air outlet diffuser.
Conventional elbows are available with a pre-formed pipe bend, most commonly 45.degree. and 90.degree. bends. The inlet head orientation of the elbow must be adjusted to match the run-in angle of the branch conduit at the connection point. After the register box has been inserted into the pre-formed ceiling opening and attached to the joists, the crimped neck of the elbow is then inserted into the female coupling collar of the register box so that the elbow inlet head is properly aligned with the branch conduit. The branch conduit is then fitted onto the elbow inlet head and the neck of the elbow is mechanically attached to the sheet-metal female coupling collar on the register box, typically by sheet-metal screws or banding straps. The sheet-metal union is then sealed with a caulking compound and/or duct tape.
Most building codes require the elbow to be constructed of a rigid material, such as galvanized sheet-metal steel, in order to avoid a sharp transition in the air flow passage and to preserve the cylindrical profile and cross-sectional flow area of the branch conduit. Additionally, the coupling of the elbow to the register box must permit horizontal swiveling of the elbow so that its inlet head section can be properly aligned with the branch conduit.
It will be appreciated that the on-site attachment of the elbow onto the register box requires an additional entry into the attic by a sheet-metal craftsman to inspect the structural integrity of the sheet-metal union and to seal leaks at the union. The mechanical attachment of the elbow to the register box is made difficult when carried out on-site within an attic space because of uncomfortable working conditions, including for example insufficient lighting, excessive heat during the summer months and excessive cold during the winter months.
A register box having an air inlet opening of a predetermined air flow area is normally supplied with conditioned air delivered by a branch conduit having the same effective air flow area. Consequently, a particular elbow having either a 45.degree. bend or a 90.degree. bend and the same effective air flow area is required for use in that particular combination. In conventional installations where the elbow and register box are connected together on-site, the installation crew must maintain an inventory of appropriately sized elbows and register boxes which must be correctly matched for a particular branch connection. It will be appreciated that the installation of the register box and elbow could be expedited and parts inventory could be simplified by pre-assembling the elbow onto the register box and stocked as a matched set item.
A further limitation imposed by on-site attachment of the elbow onto the register box is that in some attic locations, the head room is severely limited with only minimum overhead clearance being provided to accommodate the vertical projection of the elbow. In such installations, the lack of adequate head room access around the elbow and the female register collar makes branch alignment, mechanical attachment, sealing, inspection and repair difficult and in some cases, impossible.
It will be appreciated that the difficulties and limitations discussed above could be avoided by using a pre-fabricated register box and rotatable elbow combination. Such a matched register box and elbow fitting, coupled together by a mechanically stable, reliable seal, could be set up in a pre-cut ceiling opening and connected to a branch conduit quickly and reliably, even in low head room installations.
Conventional elbow fittings, capable of rotation on a register box, fail to meet the requirements discussed above. For example, U.S. Pat. No. 3,290,066 to Primich et al discloses a sheet-metal elbow fitting that is coupled to a sheet-metal air duct panel. The lower end of the elbow fitting is provided with multiple, radially projecting tabs that project from a seating and sealing bead. The end of the elbow is inserted into a panel opening, and the tabs are bent upwardly against the inner sheet-metal sidewall of the air duct panel. The elbow appears to be rotatable within the duct opening, and the union is sealed by compression of the air duct sidewall panel between the sealing bead and the tabs.
The seal integrity and structural integrity of the tab coupling arrangement of Primich et al are compromised, since a tight sealing fit of the tabs sufficient to provide a good air seal would be too tight to allow free rotation of the elbow relative to the duct panel. On the other hand, if the tab engagement is made loose enough to permit free rotation of the elbow, the air seal would be interrupted by annular air gaps, and the coupling might not be strong enough to support a hanging load, and the elbow could easily separate from the duct panel.
Consequently, there is a continuing interest in providing a rotary metal-to-metal seal between a sheet-metal conduit, such as an elbow, and a flat sheet-metal panel, such as the air inlet panel of an air distribution register box, which permits swivel adjustment and provides a good air seal as well as being mechanical stable to support the hanging load of the branch conduit.